1. Field of the Invention
The present invention generally relates to steel railway wheels, and especially those formulated to resist spalling caused by martensite transformations in the steel that constitutes the tread and/or flange regions of such wheels. Spalling in these wheel regions causes several problems. For example, spalling of the wheel tread will cause the wheel itself to have flat spots and the quality of xe2x80x9cout-of-roundnessxe2x80x9d. Moreover, when railway wheels experience spalling, surface cracks tend to propagate from spalled areas and cause pieces of the martensite steel to detach from the wheel, especially as the spalled area suffers rolling contact fatigue. These wheel defects also increase wheel/rail dynamic forces that produce consequential damage such as broken rails and accelerated track deterioration.
2. Description of the Prior Art
Steel railway wheels wear out as a result of normal usage. They are also prematurely removed from service as a result of spalling. Spalling occurs in railway wheel tread and/or flange regions as a result of metallurgical transformations caused by the heat generated when a train""s wheels skid during brake application. In effect, these skids produce local heating to temperatures above 1300xc2x0 F. (704.4xc2x0 C.). These high temperatures produce metallurgical transformations in small spots of the steel in the tread and/or flange regions of such wheels. These spots transform to martensite when they cool. The resulting brittle material then cracks and falls away. Again, spalling takes place in addition to the xe2x80x9cnormalxe2x80x9d wear experienced by railway wheels.
The railroad industry has dealt with normal wear/spalling of its wheels in three general ways: (1) machining of tread and flange surfaces, (2) scrapping the wheel and (3) imparting improved metallurgical properties to those steels from which railway wheels are made. As far as scheduled and unscheduled machining of railway wheels are concerned, it should be noted that, since normal wear/spalling of railway wheels has certain safety implications, these matters are the subject of governmental regulation. In the United States for example, the Federal Railroad Administration (xe2x80x9cFRAxe2x80x9d) has promulgated various regulations concerning the dimensions of various parts of a railway wheel""s profile. Many of these regulations express themselves in terms of the height and width of a railway wheel""s flange.
For example, these regulations call for new (or newly machined) wheel flanges to have a height of {fraction (16/16)}""s inches (i.e., 1 inch) and a width of {fraction (21/16)}""s inches (i.e., 1{fraction (5/16)} inches). A railway wheel is considered to be in violation of FRA regulations if the height of its flangexe2x80x94as measured from the crown of the tread surface of the wheelxe2x80x94reaches {fraction (24/16)}""s inches (i.e., 1xc2xd inches), or if the width of the wheel flange reaches {fraction (15/16)}""s inches. If a wheel reaches either of these states of wear, it should be machined to the required dimensions or scrapped. Those skilled in the railway wheel maintenance arts will appreciate that in order to achieve these dimensions in a worn wheel, a great deal of the wheel metal is machined awayxe2x80x94and hence, xe2x80x9cwastedxe2x80x9d. This waste has a very direct bearing on a wheel""s useful life. Hence, many machining procedures have been employed to minimize such waste. For example, U.S. Pat. Nos. 4,134,314 and 4,711,146 teach several wheel reprofiling machining techniques that serve to bring railway wheels back into compliance with regulations with minimum waste of wheel tread and flange material.
Ideally, the steel from which railway wheels are made would have high levels of at least two general properties. They would be highly wear resistant; and they also would be highly heat-crack resistant. Unfortunately, these two properties have certain contrary metallurgical aspects, especially in the context of railway wheel exposure to the heat generated by heavy braking situations. The first metallurgical problem arises because, in order to enhance its wear resistance, the hardness of the steel must be raised. Unfortunately, increased hardness in a steel usually implies decreased spall resistance. On the other hand, making a steel more spall resistant usually implies that the steel will be less hard, and hence less wear resistant. Moreover, both of these properties (wear resistance and spall resistance) must be achieved without greatly sacrificing the pearlitic structure that imparts the quality of wear resistance to a steel.
Generally speaking, increased hardness can be brought about through addition of certain alloying elements (in certain concentrations) to a steel formulation. For example, when wear resistance is the more desired property, high carbon steels having carbon contents ranging from about 0.65 to about 1.0 weight percent are employed. Such steels are especially hard and, hence, especially wear resistant. Such steels are not, however, particularly spall resistant.
Their loss in spall resistance generally follows at least in part from the fact that martensitic crystalline structures (or bainitic crystalline structures) are more likely to be produced in those railway wheel steels alloyed to gain greater hardness. These martensite crystalline structures are produced when frictional heat is imparted to railway wheel tread/flange areas in braking situations where wheel slide takes place. Such heat is often sufficient to raise temperatures of the tread/flange steel to austinite-producing levels in those local regions known as xe2x80x9chot spotsxe2x80x9d. Thereafter, because the rest of the railway wheel serves as a heat sink, hot spot temperatures are quickly lowered to martensite-forming levels. Thus, in a braking situation, local areas of the tread and/or flange are transformed from pearlite to austenite to martensite as their steel rapidly heatsxe2x80x94and rapidly cools.
Viewing the overall hardness versus heat-cracking resistance problem from the spalling resistance point of view, one finds that other alloying materials (and/or other concentrations of certain commonly employed alloying materials such as carbon) have been added to (or, in the case of carbon, reduced) certain steel formulations for the specific purpose of imparting spall resistant qualities to railway wheels. For example, medium carbon steels having carbon contents ranging from about 0.45 to about 0.55 weight percent have proved to be more spall resistant than the previously noted harder steels having 0.65 to 0.85 carbon concentrations. It also has been found that many of the other alloying materials (and/or different concentrations of identical alloying materials, e.g., the different carbon concentrations noted above) tend to have unacceptably low wear resistance. Thus, this wear resistance versus spall resistance problem has a certain dilemmatic quality that has for many years thwarted the industry""s attempts to extend the useful life of railway wheels.
Those skilled in this art also will appreciate that spalling has proven to be the more intractable aspect of the wear resistance versus heat crack resistance dilemma. This generally follows from the fact that normal wear is somewhat predictable, and gradual, in nature. Heat producing wheel skids on the other hand are relatively unpredictable. Worse yet, spalling tends to produce damage that is much more immediate and much more severe in nature. Nonetheless, most prior art railway wheel steel compositions tend toward satisfying railroad industry requirements for greater wear resistance, while xe2x80x9csilentlyxe2x80x9d conceding that spalling due to heat cracking caused by wheel skids will be dealt with by: (1) physically machining railway wheel tread/flange regions on a scheduled basis to meet the wheel flange dimension requirements previously noted, or (2) by machining heavily spalled wheels on an xe2x80x9cas neededxe2x80x9d basis, or (3) by simply scrapping the wheel.
To some extent, the patent literature reflects the railway industry""s attempts to deal with the wear resistance vs. heat crack resistance dilemma. For example, U.S. Pat. No. 5,533,770 (xe2x80x9cthe ""770 patentxe2x80x9d) teaches certain steel formulations that produce particularly hard (and, hence, particularly wear resistant) railway wheels. These formulations are characterized by their specific ratios of carbon to chromium to nickel. They also are characterized by a specific upper threshold for their silicon content and their low upper thresholds for phosphorus and sulfur. These steels are disclosed as having, in percent by mass, the following compositions:
Preferably, these steel formulations also are sequentially subjected to certain physical conditions during their overall manufacture in order to further improve their hardness. For example, they are subjected to: (1) hardening at 850xc2x0 to 900xc2x0 C., (2) quenching at room temperature at about 20xc2x0 C., (3) annealing at 600xc2x0 to 680xc2x0 and (4) slow cooling to room temperature at about 20xc2x0 C. These physical steps are all taken in order to enhance the steel""s wear resistant properties. Unfortunately, these formulations and cooling procedures do not impart particularly good heat-cracking resistance properties in the wheels made from them.
Similarly, Japanese Laid-Open Patent Application 57-143465 (xe2x80x9cJapanese Laid Open ""465 Applicationxe2x80x9d) discloses wear-resistant railway wheel steels having fine pearlitic structures. They consist of 0.55 to 0.80% C, 0.40 to 1.20% Si, 0.60 to 1.20% Mn, 0.20 to 0.70% Cr, with the remainder being iron (and trace impurities). The hardenability of the resulting steels is very high. Here again however, such steels have proven to be inclined toward heat-cracking as a result of martensitic transformations in heavy braking situations.
U.S. Pat. No. 5,899,516 (xe2x80x9cthe ""516 patentxe2x80x9d) is of particular interest with respect to the present patent disclosure because it discloses railway wheels made from steels that are specifically designed to overcome the heat-cracking problems associated with the steels described in the above-noted Japanese Laid-Open ""465 Applicationxe2x80x94while still providing good hardenability properties in such steels. The steels disclosed in the ""516 patent have the following compositions:
Moreover, the manufacturing processes used to produce railway wheels made from these steels include some very specific quenching operations. These quenching operations are intended to interrupt cooling of the steel in a railway wheel""s tread region before the steel""s cooling curve drops to the steel""s martensite forming conditions. Indeed, these quenching operations interrupt cooling of the steel before the cooling curve drops to the pearlitic transformation conditions associated with these steel compositions. As a result of these interruptions in the cooling of this steel during the wheel""s manufacture, a particularly fine pearlitic structure is imparted to the steel without the steel experiencing either a martensitic transformation or a bainitic transformation. The ""516 patent also teaches interruption of its cooling operation after the cooling curve has passed through the steel""s pearlite transformation region, but before said curve descends to the steel""s martensite transformation region. Thus, the steels taught by the ""516 patent have fine pearlitic structures and nicely avoid martensitic transformation conditions that might otherwise be encountered during the manufacture of these steelsxe2x80x94and the wheels made from them. Unfortunately, however, many martensite transformation conditions produced by the heat generated by heavy braking conditions do not coincide with the martensite transformation conditions that can be avoided in highly controlled manufacturing processes such as those disclosed in the ""516 patent.
However, before delving into applicants"" methods for producing railway wheels that are more resistant to the martensite transformations that result from heavy braking situations, a few general observations about steel transformations in general, and martensite transformations in particular, may be helpful. Those skilled in the steel making arts will appreciate that martensite transformations take place when a steel having an austenite structure transforms to a steel having a martensite structure as a result of a rapid cooling of an austenite steel. It might also be emphasized at this point that martensite can not be directly produced from a steel whose metallurgical structure is pearlitic in nature. Next we note that a martensite transformation from austenite does not involve any change in chemical composition. That is to say there is no nucleation followed by growth in a martensite transformation product. Rather, small discrete volumes of the parent austenite solid solution, very suddenly, change to the martensite crystal structure. Indeed, the time of formation of a single plate of martensite in iron-nickel alloys can be on the order of about 7xc3x9710xe2x88x925 seconds. Such very short transformation times have a considerable bearing on applicants"" inventive concept. Therefore, a great deal more will be said about the implications of these short martensite transformation times in subsequent parts of this patent disclosure.
For now however, a few other observations about martensite are in order. For example, it should be understood that a martensite transformation progresses only while the steel is cooling (that is to say that more and more discrete volumes of the parent austenite solid solution transform as the steel cools). It also should be appreciated that martensite transformations cease if cooling is interrupted. Thus, a martensite transformation is independent of time and depends for its progress only on decrease in temperature. It might also be noted at this point that the term Ms is applied to the temperature of the start of a martensite formation; similarly, the term Mf indicates the temperature of the finish of a martensite transformation. It also should be noted that the amount of martensite formed per degree of decrease in temperature is not a constant (i.e., the number of martensite crystalline units produced at first is small, increases rapidly as the temperature continues to decrease, but eventually decreases again).
Those skilled in the steel making arts also will appreciate the following related points:
(1) Austenite is an allotropic form of iron called xe2x80x9cgammaxe2x80x9d with carbon in solution. Austenite transforms to various other products (including martensite) on cooling below 723xc2x0 C. The nature of these other products depend to a large degree upon the rate of cooling of the austenite.
(2) Ferrite (virtually pure iron) has an upper limit of existence that is lowered progressively to about 723xc2x0 C. as the steel""s carbon content increases up to 0.83%.
(3) Cementite, iron carbide Fe3C, is one of the products that can be precipitated when austenite cools.
(4) Pearlite is a eutectoid comprised of a laminated structure of ferrite and cementite. Pearlite is formed by transformation of austenite upon cooling. The fineness of a pearlite""s laminated structure is determined in large part by the rate of cooling. The lamellar structure of ferrite and cementite in pearlite produces its highly desired quality of wear resistance.
Thus, even though a great deal is known about martensite transformations, the fact remains that such transformations are responsible for a great deal of the accelerated wear of railway wheels through spalling of railway wheel tread/flange regions as a result heavy braking. It is therefore an object of this invention to provide steels for railway wheels that have increased spalling resistance by virtue of their ability to avoid martensite transformation conditions.